In recent years, a secondary battery module having high energy density is developed as a power source of an electric vehicle or the like. Especially for on-board use, since loads from various directions are expected, and a large current flows, connection between metal members as a current path is required to have strength and to be reduced in electrical resistance.
For example, a cylindrical secondary battery is formed to have a sealed structure by housing a power generation element in a battery case of a tubular shape having a bottom, by injecting electrolyte and by sealing the opening of the battery case with a battery cover. The power generation element includes positive and negative electrodes, and for example, the positive electrode is electrically connected to the battery cover. A safety valve unit integrated with the battery cover by caulking or the like is provided between the positive electrode and the battery cover. The battery cover and the safety valve unit are caulked to a peripheral edge part of an opening provided at one end side of the battery case through a gasket.
The battery cover is made of, for example, stainless, and the safety valve unit is made of, for example, aluminum.
A method is known in which in order to increase the strength of joining between the battery cover and the safety value unit and to reduce the electrical resistance, a laser is irradiated to an end of the safety valve unit, and the safety valve unit is welded to the battery cover (see, for example, PTL 1). This prior art literature discloses that the safety valve unit is melted by the laser irradiation, and flowing aluminum-based metal and iron-based metal are fused and are firmly joined.